[Dune] --disable-documentation.

[Robert Kloefkorn]

Hello all,

I suggest to switch the default for building of documentation to 
disabled. When using the tar balls from the web the docu should have 
been build already. For people using the trunk we have the docu on the 
web. Therefore, the most common case is not to build the docu. Therefore 
I suggest to switch from --disable-documentation to 
--enable-documentation for people who really want that.

Cheers

R